The trouble with the way the media report developments in the labour market from one month to the next is that we don't get a sense of the major shifts that occur over time.

So today let's take a much longer view, examining the trends over, say, the two decades from 1993 to 2013. We'll do so with help from an article by Professors Roger Wilkins and Mark Wooden, of the Melbourne Institute, published in the latest issue of the Australian Economic Review.

Note that this period covers most of the continuous economic upswing since the severe recession of the early 1990s. So most of the trends are reasonably good. It's true, however, that we were knocked off track briefly by the global financial crisis of 2008-09 and in more recent years have suffered a slow deterioration in unemployment as the economy makes heavy weather of the end of the mining investment boom.

But that's getting ahead of the story. Actually, it's such a long story that today I'll limit it to the side that gets least attention from the media, changes in the supply of labour. It's best measured by changes in the "participation rate" – the proportion of the population of working age who are participating in the labour market by making their labour available, either by having a job or actively seeking one.

Taken overall, the "part rate" increased pretty strongly until 2008, when it began falling back. But all of this overall increase is explained by the increased participation of women, particularly those of prime age, 25 to 54.

There's been a long-term slow decline in the participation of men. It's explained partly by young males staying longer in the education system but mainly by older workers retiring earlier – voluntarily or otherwise.

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But here's where the story gets complicated. The trend to earlier retirement turned around at about the turn of the century, with participation by both men and women aged 55 and over rising significantly.

Got that? Now try this. Although more people are retiring later, the part rate reached a peak in 2008, has fallen since then and is likely to continue falling for years yet. Why? Because of the ageing of the population.

The trick is that even if the part rate is now rising in older age groups, population ageing means that an ever-rising proportion of the labour force is in those older age groups, whose rates of participation will always be a lot lower than the rates for people of prime age.

To show the significance of this ageing effect, the authors calculate that if the age structure of the population in 2013 was the same as in 1993, the overall part rate would be 2.2 percentage points higher than it actually is.

However, we do have some scope to moderate this demography-driven decline in participation. Wilkins and Wooden note that the rates of participation for 55 to 64-year-olds are between 7 and 14 percentage points higher in New Zealand than they are in Oz. If the Kiwis can do it, what's to stop us doing it?

The part rate covers the quantity of people willing to supply their labour, but there's also the question of changes in the quality of the labour being supplied.

The past 20 years have seen a big improvement in the skills – education and training – of the labour force, with the proportion of university graduates more than doubling from 12 per cent to 28 per cent. The proportion with any post-secondary qualification rose from less than 46 per cent to more than 62 per cent.

By the way, it's likely that the continuing rise in women's participation is largely explained by the dramatic increase in females' academic attainment. The higher men and women's level of education, the greater the likelihood they'll be in the labour force – exploiting the commercial value of their skills – and the less the likelihood of them being unemployed.

Of course, another part of the labour supply story is immigrant workers. Immigration has long been a major source of additional labour and today accounts for more than a fifth of the labour force. What's changed is that throughout the last century most migrants came on permanent visas, whereas today most come on temporary visas.

In March last year there were almost 900,000 people on temporary visas with work rights, including more than 200,000 on "457 visas" for skilled people and about 370,000 on student visas. If all these people actually participated they'd amount to 7 per cent of the labour force, the authors estimate.

Separate to this were almost 650,000 people on the special visas for New Zealanders, some of whom will prove to be only temporary residents. (Don't forget Aussies have reciprocal rights to work in Kiwiland.)

We now grant about 125,000 457 visas a year and about 100,000 student visas a year. This compares with about 130,000 old-style permanent visas a year to skilled immigrants, many of which are given to people already here on temporary visas.

The authors observe that the shift towards temporary migration has probably had a big effect on the labour market.

"The availability of a flexible skilled immigrant workforce that can respond to changes in labour demand relatively quickly is likely to have improved the operation of the labour market, especially from an employer perspective," they say.

Oh. Yes. To me the main drawback is not so much that employers may not try hard enough to find local workers to fill jobs, or that the availability of this external supply may limit to some extent the rise in skilled wages, but that it reduces employers' incentive go to the bother of training young workers.

Still, we mustn't forget that, these days, the economy is run for the benefit of business, not the rest of us.